Without limiting the scope of the present invention, its background will be described with reference to perforating a hydrocarbon bearing subterranean formation with a shaped-charge perforating apparatus, as an example.
After drilling the section of a subterranean wellbore that traverses a hydrocarbon bearing subterranean formation, individual lengths of metal tubulars are typically secured together to form a casing string that is positioned within the wellbore. This casing string increases the integrity of the wellbore and provides a path through which fluids from the formation may be produced to the surface. Conventionally, the casing string is cemented within the wellbore. To produce fluids into the casing string, hydraulic openings or perforations must be made through the casing string, the cement and a distance into the formation.
Typically, these perforations are created by detonating a series of shaped-charges located within one or more perforating guns that are deployed within the casing string to a position adjacent to the desired formation Conventionally, the perforating guns are formed from a closed, fluid-tight hollow carrier gun body adapted to be lowered on a wire line or tubing conveyed into the wellbore. Disposed within the hollow carrier gun body is a charge holder that supports and positions the shaped-charges in a selected spatial distribution. The shaped-charges have conically constrained explosive material therein. A detonating cord that is used to detonate the shaped-charges is positioned adjacent to the rear of the shaped-charges. The detonating cord can be activated electronically or mechanically when the perforating gun has been positioned in the wellbore.
In such closed, fluid-tight type gun bodies, the explosive jets produced upon detonation of the shaped-charges penetrate the hollow carrier gun body before penetrating the casing wall of the wellbore and the adjacent formation. To reduce the resistance produced by the hollow carrier gun body and increase the depth of perforation penetration into and the formation, the perforating gun body may be provided with scallops or other radially reduced sections such as bands that leave relatively thin wall portions through which the explosive jets pass. The scallops in the hollow carrier gun body must be positioned in a spatial distribution that corresponds to the spatial distribution of the shaped-charges held within the gun body by the charge holder.
It has been found, however, that the reduction in thickness of the carrier gun body limits the strength of the perforating guns. Thus, to perforate in certain high pressure and high temperature wellbores, perforating guns of a given outer diameter have relatively increased wall thickness. Further, use of a carrier gun body with increased wall thickness reduces the available volume within the carrier gun body which necessitates the use of smaller shaped-charges. Likewise, use of a carrier gun body with a thick wall limits the penetration depth of the perforations into the formation. In either case, the performance of such perforating guns is diminished. Of greater concern, are the cracks that often result in the carrier gun body extending from the perforations in the body caused by the shaped charges during perforation. These cracks have the potential to cause failure of the carrier gun body and even separation from the gun string. Additionally, sharp projections of carrier gun body tend to extend from the outer wall after perforation. These projections can hang-up upon pulling the tool from the hole.
A need has therefore arisen for a perforating apparatus that is operable for use in high pressure and high temperature wellbores that does not require a carrier gun body with increased wall thickness. A need has also arisen for such a perforating apparatus that is operable for use in high pressure and high temperature wellbores that does not require a carrier gun body with reduced scallop depth. Further, a need has arisen for such a perforating apparatus that is operable to achieve enhanced perforating performance in high pressure and high temperature wellbores.